3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS, and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. The Evolved UMTS Terrestrial Radio Access Network consists of base stations called enhanced NodeBs (eNBs or eNodeBs), providing the E-UTRA user plane and control plane protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the Evolved Packet Core (EPC).
The eNB hosts functionalities such as Radio Resource Management (RRM) radio bearer control, admission control, header compression of user plane data towards serving gateway, routing of user plane data towards the serving gateway.
Today, user equipments (UEs), such as mobile phones, typically support other wireless technologies such as Wireless Local Area Networks, commonly referred to as WLAN, in addition to the cellular standards. As a means to improve network capacity in future networks, WLAN is expected to be an integral part. That is, WLAN will be regarded as just another radio access technology, so that handover can be made to WLAN without the user necessarily noticing that the service is no longer being carried by 3GPP technologies like WCDMA or LTE. Mobile operators are today mainly using WLAN to offload traffic from the mobile networks, but the opportunity to improve end user experience regarding performance is also becoming more important previous WLAN deployments have generally been totally separate from mobile networks, and can be seen as “non-integrated.” The usage of WLAN is driven due to the free and wide unlicensed spectrum, and the increased availability of WLAN in mobile terminals like smart phones and tablets. The end users are also becoming more and more at ease with using WLAN for example at offices and homes.
Presently, there is insufficient coordination and control of the combined cellular and WLAN network, because the WLAN network is still not sufficiently tightly integrated with the cellular network. Improved WLAN integration with the cellular network is emerging as a good way to improve the end user experience further.
3GPP has studied better ways to integrate LTE and WLAN, in particular for operator-deployed WLANs, so that traffic from WLAN-enabled UEs can be offloaded to WLAN from an eNB (3GPP terminology for a base station in an LTE system) and vice versa. These studies are part of the development of the Release 12 and Release 13 versions of the 3GPP standards for wireless networks. In early iterations of these standards, integration of LTE and WLAN communications was achieved through semi-static policy setting, via the specification of an Access Network Discovery and Selection Function (ANDSF) and/or offload thresholds communicated by the eNB to the mobile (either in a broadcast or dedicated fashion). The pre-Release 13 WLAN interworking is based on an architecture like that shown in FIG. 1, where the data may be routed via a Public Data Network (PDN) Gateway (P-GW) to WLAN, using the GPRS tunneling protocol (S2a) of the SA2 specification.
A 3GPP Release 13 study item entitled “Multi-RAT Joint Coordination” has been initiated in 3GPP TSG RAN3 [3GPP TR 37.870]. (“RAT” refers to a “radio access technology.”) For the 3GPP-WLAN coordination part, it has been agreed to focus on non-integrated 3GPP/WLAN nodes (i.e., scenarios where the WLAN access point is not built into the eNB itself), since integrated nodes are a matter of implementation.
Among the requirements of the study item, as specified in the 3GPP document 3GPP TR 37.870, is the investigation of potential enhancements of radio-access network (RAN) interfaces, and procedures to support the joint operation among different RATs, including WLAN. It has also been agreed that coordination involving WLAN and 3GPP is a priority of the study item. A key to this coordination is the specification of an interface between the E-UTRAN and WLAN.
A key functionality envisioned for this interface, referred to so far as the “Xw” interface, is the support for traffic steering from LTE to WLAN via the reporting of different sets of information from WLAN to the eNodeB so that educated steering decisions can be taken. Also, 3GPP has recently approved a RAN2 work item on full network controlled 3GPP/WLAN traffic steering and aggregation [RP-150510 (ftp://ftp.3gpp.org/tsg_ran/TSG_RAN/TSGR_67/Docs/RP-150510.zip)], and thus new functionalities of the Xw interface can be envisioned.
The Xw interface is terminated in the eNB on the LTE side, and in a logical node called WT (WLAN Termination) on the WLAN side. The WT can physically reside in an WLAN access point (AP) or in a WLAN access controller (AC), and is assumed to have IP connectivity to the relevant WLAN nodes. Possible realizations of the basic architecture and protocol stack configurations related to the Xw interface are depicted in FIG. 2 and FIG. 3, respectively. Note that in the figures the AP/AC has been assigned the role of the WT for the sake of clarity, but the WT node can be another node in the WLAN. Also, one WT can be the interface point for several APs/ACs.
The 3GPP work item on tighter interworking (RP-150510) consists of two parts, which offer separate solutions for a general problem of how to exploit efficiently the two widely deployed RATs that operate on separate spectrum. The first part of the Release 13 solution involves fully network controlled offloading, which is a continuation of Release 12 WLAN interworking where the interworking is enhanced with WLAN measurement reporting and traffic steering commands. The second part deals with aggregating the traffic of a given flow (bearer) between WLAN and 3GPP, on a per packet basis.
In the aggregation solution, WLAN is aggregated with LTE on the Packet Data Convergence Protocol (PDCP) layer, reusing the Release 12 solution for Dual Connectivity. The only interfaces towards the core network are those of LTE, i.e., the S1-MME and S1-U from eNB to MME and S-GW, respectively. New interfaces for the control and user planes between eNB and WLAN node need to be considered for the non-collocated case. Thus, more seamless mobility, without the heavy core network signaling of the interworking solution, can be expected. FIG. 4 presents a WLAN aggregation protocol stack based on 3C Release 12 dual connectivity, for the case of non-collocated eNB and AP.
Following are several requirements for LTE/WLAN solutions:                Solutions shall consider only WLAN nodes deployed and controlled by operators and their partners.        Solutions for aggregation should build upon Release-12 LTE dual connectivity architecture.        Solutions shall improve mobility to/from WLAN while minimizing the core network signaling.        Solutions shall improve network control of WLAN offload.        Solutions shall improve overall UE throughput by using both cellular and WLAN access.        
The following objectives related to aggregation should also be considered:                Specify RAN and WLAN protocol architecture of LTE-WLAN aggregation at the UE and network side based on Release-12 LTE Dual Connectivity solutions 2C and 3C.        Specify solution for user plane aggregation at the PDCP layer based on Release-12 LTE Dual Connectivity allowing both per packet (i.e. per PDCP PDU as in Dual Connectivity split bearer) and per bearer offloading. For the case of per packet offloading, downlink should be specified with higher priority than uplink.        Specify RRC enhancements for network-controlled activation and de-activation for aggregation based on Release-12 LTE Dual Connectivity.        Specify solutions for addition, removal, and change of WLAN links while being connected to the same eNB.        Specify UE WLAN measurement reporting for aggregation and inter-working enhancements.        
Among other things, these requirements and objectives imply that the 3GPP will specify the reporting of WLAN-side measurements by the UE to the eNB, and that the eNB will be in control of the mobility. The latter implication results, in part, from the fact that in WLAN networks, mobility is handled by the “stations,” i.e., the UEs, while in LTE the mobility of UEs is handled by the eNBs.